1. Field of the Invention
The present invention relates to an illuminating device and a method of calibrating an illuminating device.
2. Description of the Related Art
Conventionally, an image processing apparatus is used which is provided with an illuminating device having an LED (light emitting diode) for irradiating a workpiece with light and a controller for controlling the LED, and in which an image of the workpiece is obtained by irradiating the workpiece with light by the illuminating device so as to determine a profile of the workpiece from the image of the workpiece. The controller reads from a predetermined conversion table an output current command value corresponding to a brightness command value, which is a command value of illuminance inputted from outside, and outputs the output current command value thus read to a DA converter, to thereby light the LED. However, the illuminance of the LED at the time when the predetermined current value is inputted is greatly dependent on the characteristics of the LED. For this reason, if, with respect to the inputted brightness command value, the controller outputs a uniformly predetermined output current command value to the LED on the basis of the conversion table, there are cases where the commanded illuminance cannot be obtained due to the characteristics of the LED incorporated in the illuminating device.
In the image processing apparatus, if the illuminance becomes lower than the commanded illuminance, the quantity of light becomes insufficient, in which case there are possibilities that an image of the workpiece cannot be obtained, and edges of the workpiece cannot be detected. Meanwhile, if the illuminance becomes higher than the commanded illuminance, the image of the workpiece becomes blurred, possibly making it impossible to detect edges of the workpiece. For these reasons, with the illuminating device used in the image processing apparatus, controlling of the light source such that the commanded illuminance can be obtained becomes an extremely important issue.
Accordingly, an illuminating device has been developed in which the conversion table is calibrated before shipment in correspondence with the characteristics of the incorporated LED (e.g., JP-A-2004-220834). The illuminating device of JP-A-2004-220834 at a shipment stage is capable of irradiating with a commanded illuminance irrespective of the characteristics of the incorporated LED.
However, since the LED undergoes a decline in illuminance with the lapse of time, with the illuminating device of JP-A-2004-220834, after the lapse of sufficient operating time after shipment the conversion table becomes incapable of coping with the LED whose illuminance has dropped, possibly making it impossible to irradiate with the commanded illuminance. Accordingly, with the illuminating device of JP-A-2004-220834, in the case where sufficient operating time has elapsed after shipment, it is desirable to recalibrate the conversion table in correspondence with the decline in the illuminance of the LED. However, in the recalibration of the conversion table by the illuminating device of JP-A-2004-220834, it is necessary to perform time-consuming measurement of illuminance a number of times. For this reason, there is a problem in that much time is required in the case of recalibrating the conversion table by the illuminating device of JP-A-2004-220834.